The invention relates to a colour display tube comprising a display window with a circumferential upright edge and corner areas, a colour selection electrode comprising corner sections to which suspension means are coupled, which is suspended from supporting elements, having free end portions with an axis of symmetry, coupled to the corner areas. The invention also relates to a supporting element for use in a colour display tube.
A colour display tube as described in the opening paragraph is disclosed in U.S. Pat. No. 4,763,039. The colour display tube according to this specification is provided with a colour selection electrode, which is suspended in the corners of the display window. In this corner suspension system, the supporting elements, connected in the corners of the upright edge of the display window, are coupled to the suspension means of the colour selection electrode. The suspension means comprise a resilient element and a part with an aperture for receiving the spherically curved free end portion of the supporting elements.
The colour display tube described in U.S. Pat. No. 4,763,039 is provided with a colour selection electrode, to ensure that electron beams coming from three electron guns, mounted in a neck portion of the tube, only excite one colour of electroluminescent material on the inner side of the display window. This colour selection is achieved by applying, for instance, a shadow mask, comprising a large number of apertures, which are generally arranged in either a slotted pattern or a dotted pattern. If the colour selection electrode is not stably positioned in the colour display tube, small deviations of its position lead to a deterioration of the picture quality. When the colour selection electrode is shifted slightly, the shadowing effect of the colour selection electrode changes and, consequently, the electron beams do not hit the appropriate electroluminescent material on the display window. This misregistration causes a lack of the corresponding colour, or even worse, the wrong colour of electroluminescent material is excited. These misregistrations cause discolouration of the colour display tube that lead to a deterioration of the picture quality.
One of the items determining the stability of the colour selection electrode is the insertion process. It appears that positioning the colour selection electrode before it is inserted in the display window is a critical process. The ever increasing demand for a very accurately defined position of the colour selection electrodes, especially in wide screen tubes and in tubes with a flat or almost flat outer surface of the display window, causes the supporting elements as described in U.S. Pat. No. 4,763,039 to have the disadvantage that this demand is no longer met.
It is an object of the invention to provide a colour display tube with a colour selection electrode whose suspension system has been improved with respect to the type described in the opening paragraph, which strongly facilitates the insertion process of the colour selection electrode and thereby diminishes the registration errors on the display window.
According to the present invention, this object is achieved by means of a colour display tube, which is characterized in that each free end portion has a smoothly shaped centring tip in the direction of the axis of symmetry.
The invention is based on the insight that the insertion process of the colour selection electrode will gain in robustness when the free end portions of the supporting elements are provided with a centring tip. When the colour selection electrode is mounted in the display window, the free end portions are in general received by the apertured part of the suspension means. In order not to damage the colour selection electrode, and more particularly the suspension means, the colour selection electrode has to be positioned with great accuracy with respect to the free end portions of the supporting elements. This makes the insertion process very critical and in an appreciable number of cases the insertion process will fail. Applying a free end portion with a centring tip overcomes this disadvantage. The tip of the free end portion needed for finding its position in the apertured part of the suspension means is smaller than the diameter of the free end portions of prior art colour display tubes, thereby facilitating the insertion of the colour selection electrode into the display window.
The positional stability of the colour selection electrode is increased when the friction between the free end portion and the suspension means is reduced. This is achieved by decreasing the diameter of the free end portion, as is described in the unpublished European Patent Application EP 99203977.6. When the diameter of the free end portion becomes smaller, evidently also the aperture in the suspension means becomes smaller. This makes positioning of the colour selection electrode with respect to the supporting elements more difficult; this problem is solved by providing the free end portions with a centring tip.
Furthermore, it is of essential importance that the surface of the free end portion is smooth. Here, smooth has the meaning of mathematically differentiable; that is to say the surface has no sharp edges. For instance, in U.S. Pat. No. 4,387,321 a free end portion is disclosed that is shaped such that it can serve as a centring tip, but it does not have a smooth surface; its shape is conical with a flat end. A smoothly shaped free end portion has a number of advantages over the free end portions described in U.S. Pat. No. 4,387,321. Firstly, the centring properties are better because a smooth centring tip more easily slides into the aperture, whereas a centring tip with a flat end can collide with the edges of the aperture in the suspension means. Secondly, a smooth centring tip does not cause loose particles, while a centring tip with sharp edges easily scratches along the aperture of the suspension means during insertion of the colour selection electrode. Thirdly, the friction of a smooth centring tip in the aperture of the suspension means is lower, resulting in a better positional stability.
A preferred embodiment is characterized in that the free end portion further comprises a centre of gravity and a geometrical centre situated on the axis of symmetry at a larger distance from the upright edge than the centre of gravity. This means that, seen along the axis of symmetry, the geometrical centre is oriented more towards the end of the free end portion than the centre of gravity. So, the major part of the mass is directed away from the tip of the free end portion, which is a suitable situation for realizing a centring tip on the free end portion.
In a further embodiment, the free end portion is convex in shape. A convex free end portion is easy to manufacture and has a simple, smooth structure, which is very simple to centre in the aperture of the suspension means. It is to be noted that convex in this context means that the free end portion does not show any indentations; mathematically this means that it applies for any two points inside the free end portion, that also all the intermediate points on the line between said two points are inside the free end portion.
A still further embodiment is characterized in that the free end portion comprises a conical section which smoothly interconnects the substantially spherically shaped bottom part and the substantially spherically shaped centring tip, the radius of curvature of the centring tip being smaller than the radius of curvature of the base part. This shape of the free end portion has the advantage that a spherically shaped tip with a small radius of curvature is easy for centring purposes, while the bottom part is shaped so that the free end portion according to the present invention can easily be used instead of the spherically shaped free end portions according to prior art, often without changing the geometry of the apertures in the suspension means.
The invention also relates to a supporting element for use in a colour display tube according to the present invention.